1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine and a laser beam printer which forms a toner image on an ultrathin transfer material using chromatic color toner or black toner.
2. Description of the Related Art
A conventional image forming apparatus includes a conveying belt for bearing and conveying a transfer material between a photosensitive drum and a transfer apparatus. The conveying belt is wound around a plurality of rollers including a drive roller. The plurality of rollers rotates according to rotation of the drive roller, and the conveying belt rotates. Based on this configuration, U.S. Pat. No. 7,440,720 and Japanese Patent Application Laid-open No. 2001-356564 propose inventions having an attraction roller so that a transfer material is reliably attracted by a conveying belt.
The inventions described in U.S. Pat. No. 7,440,720 and Japanese Patent Application Laid-open No. 2001-356564 relate to an image forming apparatus in which an attraction roller is disposed on an upstream side in a conveying direction of the transfer material, and a separation roller is disposed on a downstream side in the conveying direction of the transfer material. According to the image forming apparatus described in U.S. Pat. No. 7,440,720 and Japanese Patent Application Laid-open No. 2001-356564, the transfer material is reliably attracted by the conveying belt from a position of the attraction roller to a position of the separation roller.
However, when toner has negative electric charge, positive electric charge cannot easily move from a conveying belt 724 to a transfer material 7, and an arborescent, abnormal image is prone to be generated at a position of a separation roller 726 (see FIG. 12A).
When the separation roller does not have a cross section which is uniform in a longitudinal direction thereof, e.g., when the separation roller has a crown shape as illustrated in FIG. 2A, this may cause a problem as follows. That is, an arborescent abnormal image is conspicuously generated at an edge of the transfer material in its width direction as compared with a central portion of the transfer material in its width direction. This is because that the edge of the transfer material corresponds to a place where separating timing of the transfer material is early, and greater creeping discharge (which causes the arborescent image) is generated at a place where the separating timing of the transfer material is earlier. Further, positive electric charge is increased on a surface of the transfer material at a place where the separating timing of the transfer material is early, and positive electric charge is not increased on the surface of the transfer material at a place where the separating timing of the transfer material is late. Therefore, positive electric charge on the surface of the transfer material is not uniformly electrified in the width direction of the transfer material.
When the transfer material is uniformly electrified, if electrification is possible such that it maintains a balance with the electric charge amount of a toner image, electric discharge is not generated at the separating portion and image failure is not caused. Actually, however, an image is changed in every page or job, the electric charge amount of a toner image in a width direction of a transfer material is not uniform in many cases, and it is difficult to uniformly electrify the transfer material to keep a balance with the electric charge amount of a toner image.
FIG. 12B is a table illustrating generating states of abnormal images in which a condition of halftone (HT), a condition of ultrathin transfer material, a condition of thin transfer material, a condition of normal transfer material and a condition of thick transfer material are taken into consideration. The generating states of abnormal images are evaluated based on measurement using a spectrodensitometer produced by X-Rite, Incorporated, and the generating states are determined based on the quality of the image density. In FIG. 12B, a state of an image is expressed by ∘, Δ and x, wherein ∘ portion excellent, Δ portion permissible but not excellent, and x portion failure. As illustrated in the leftmost column of FIG. 12B, when dot D of a halftone image is 0.6 and transfer material is ultrathin, an abnormal image is generated. When dot D of the halftone is 1.6 and transfer material is thick, an abnormal image is not generated. Image failure at a separating portion is more frequently generated when the image is of halftone, especially in the case of a highlight, and image failure is less likely in maximum image density of an engine (solid image). That is, if the design is made so that image failure is reduced in a highlight image, the image failure is reduced in all of images.
Even when an amount of toner on an entire surface of an image is uniform as illustrated in FIGS. 4A and 4C, it is difficult, in the first place, to electrify a transfer material such as to keep a complete balance with an amount of electric charge of a toner image. Even when a transfer material is uniformly electrified before a transfer material passes through a secondary transfer portion such as to keep a balance as much as possible, the transfer material is positively or negatively electrified, i.e., the transfer material is polarized positively or negatively. If a configuration at a separating portion is not uniform in its longitudinal direction, unevenness is generated in image failure at the separating portion in the transfer material in its width direction irrespective of polarity.
It is desired to provide an image forming apparatus capable of reducing image failure such as unevenness in a width direction of a transfer material that may be generated when the transfer material is separated from a transfer material conveying belt.